text: There are two major classes of devices: single-beam and double-beam. A double-beam spectrophotometer compares the light intensity between two light paths, one path containing a reference sample and the other the test sample. A single-beam spectrophotometer measures the relative light intensity of the beam before and after a test sample is inserted. Although comparison measurements from double-beam instruments are easier and more stable, single-beam instruments can have a larger dynamic range and are optically simpler and more compact. Additionally, some specialized instruments, such as spectrophotometers built onto microscopes or telescopes, are single-beam instruments due to practicality.
  Historically, spectrophotometers use a monochromator containing a diffraction grating to produce the analytical spectrum. The grating can either be movable or fixed. If a single detector, such as a photomultiplier tube or photodiode is used, the grating can be scanned stepwise (scanning spectrophotometer) so that the detector can measure the light intensity at each wavelength (which will correspond to each "step"). Arrays of detectors (array spectrophotometer), such as charge coupled devices (CCD) or photodiode arrays (PDA) can also be used. In such systems, the grating is fixed and the intensity of each wavelength of light is measured by a different detector in the array. Additionally, most modern mid-infrared spectrophotometers use a Fourier transform technique to acquire the spectral information. This technique is called Fourier transform infrared spectroscopy.
  When making transmission measurements, the spectrophotometer quantitatively compares the fraction of light that passes through a reference solution and a test solution, then electronically compares the intensities of the two signals and computes the percentage of transmission of the sample compared to the reference standard. For reflectance measurements, the spectrophotometer quantitatively compares the fraction of light that reflects from the reference and test samples. Light from the source lamp is passed through a monochromator, which diffracts the light into a "rainbow" of wavelengths through a rotating prism and outputs narrow bandwidths of this diffracted spectrum through a mechanical slit on the output side of the monochromator. These bandwidths are transmitted through the test sample. Then the photon flux density (watts per meter squared usually) of the transmitted or reflected light is measured with a photodiode, charge coupled device or other light sensor. The transmittance or reflectance value for each wavelength of the test sample is then compared with the transmission or reflectance values from the reference sample. Most instruments will apply a logarithmic function to the linear transmittance ratio to calculate the absorbency of the sample, a value which is proportional to the concentration of the chemical being measured.
  In short, the sequence of events in a scanning spectrophotometer is as follows:
  # The light source is shone into a monochromator, diffracted into a rainbow, and split into two beams. It is then scanned through the sample and the reference solutions.
  # Fractions of the incident wavelengths are transmitted through, or reflected from, the sample and the reference.
  # The resultant light strikes the photodetector device, which compares the relative intensity of the two beams.
  # Electronic circuits convert the relative currents into linear transmission percentages and/or absorbance/concentration values.
  In an array spectrophotometer, the sequence is as follows:
  # The light source is shone into the sample and focused into a slit
  # The transmitted light is refracted into a rainbow with the reflection grating
  # The resulting light strikes the photodetector device which compares the intensity of the beam
  # Electronic circuits convert the relative currents into linear transmission percentages and/or absorbance/concentration values
  Many older spectrophotometers must be calibrated by a procedure known as "zeroing", to balance the null current output of the two beams at the detector. The transmission of a reference substance is set as a baseline (datum) value, so the transmission of all other substances is recorded relative to the initial "zeroed" substance. The spectrophotometer then converts the transmission ratio into absorbency, the concentration of specific components of the test sample relative to the initial substance.